1. Field of the Invention
The present invention relates to a moisture-crosslinking PU hot-melt adhesive prepared by reacting an isocyanate-containing prepolymer with a sterically hindered polyamine having an NH functionality .gtoreq.2.
2. Discussion of the Background
Hot-melt adhesives are easy to manufacture and are useful in a wide variety of applications since they can be made without solvents and set rapidly without pot-life problems. These materials do have limitations on their use, however, because they are often only moderately stable. Hot-melt adhesives are thermoplastic systems and, therefore, the melting process is reversible. As the temperature rises, the bonded joint becomes more and more plastic until, finally, the liquid melt state is reached again. The chemical stability of hot-melt adhesives and their resistance to solvents, plasticizers, oils and chemicals is similar to other non-crosslinked resins and plastics. Resistance may be high in a particular medium, overall, however, the hot-melt adhesives can be expected to show greater swellability or even solubility as compared to crosslinked adhesives.
These fundamental weaknesses of thermoplastic hot-melt adhesives are largely overcome by the reactive polyurethane (PU) hot-melt adhesives. These materials are produced from moisture-curing polyurethane prepolymers, which were introduced into the market at the beginning of the 1980s. The PU hot-melt adhesives are initially meltable, like the conventional hot-melt adhesives, and set in accordance with the typical hot/cold mechanism. After the initial setting, these adhesives undergo irreversible crosslinking in the bonded joint within a period of a few days, as a result of chemical reaction of remaining --NCO groups with moisture. During this process, the originally thermoplastic film undergoes a transition to a thermoset state. In other words, the formerly thermoplastic hot-melt adhesive joint is now crosslinked and can no longer be melted. From the time of application to crosslinking there is a time period where the initial adhesion strength, i.e., the strength of the bonded materials directly after bonding, is relatively low. The length of this time period depends on the chemical composition of the adhesive and can range from several hours to several days.
In many applications, rapid setting of the hot-melt adhesive is necessary in order to further process the bonded articles and still maintain a short pot life. Accordingly, there have been many attempts to produce moisture-crosslinking PU hot-melt adhesives that have improved initial strength and a shorter setting time.
For example, EP-A 0 248 858 discloses that polyesters having a large proportion of aromatic dicarboxylic acid units improve the setting rate, but leads to products with a high melt viscosity, which hinders processing, and a flexibility in the fully cured state that is too low for many applications. The rapidly crystallizing polyesters disclosed in DE-A 38 27 224 suffer from many of the same disadvantages because the strong crystallization tendency leads to a perceptible change in volume during curing, which causes the adhesive film to peel from the substrate.
Accordingly, there remains a need for moisture-crosslinking hot-melt PU adhesives which have high initial adhesive strength and short setting time which overcome these disadvantages.